I have been commissioned to complete renovations of an existing installation to reconstruct one side of a semi-raised pond, and to turn something purporting to be a filtration system into an actual, working, mechanical and biological filter.

When I first went to quote for the pond reconstruction job, I was amazed by how anyone could have constructed a semi-raised pond in the way this builder did. The dimensions were about ten feet long, eight feet wide and four feet deep at the deepest point. Its okay so far, from the point of view of it being the size of your average ornamental, mixed fisheries pond in the country.

Where it fell short was that the original builder had raised the pond about two feet above ground, and instead of using a solid concrete collar, (usually in these cases I make the collar about six inches deep and nine inches wide for strength) he had used nine times plywood! Granted it was novel, but hardly going to hold up.

A combination of leaking pipework and bad workmanship meant that one side of the pond had been subjected to continuous water damage that, naturally, caused the wood to rot and eventually snap.

My solution was to make a concrete collar that fortified the wood on either side of the damage, raise it to the level of the rest of the wooden collar, and overlay the existing butyl liner with paving slabs to finish it off.

Using a mix of five to one sharp sand and cement, I used the damaged wood as shuttering, and created a concrete collar over which the existing liner could go.

After 36 hours of curing, (needed because the collar would be taking the weight of the customer when maintenance was necessary) I pulled the liner on to the collar and began setting on the pavers, using a four-to-one mix.

As the water level was intended to lap at the bottom edge of the paving, the cement between the slabs and liner was sealed with a plasticiser, after letting the muck cure off. The plasticiser produces an effective seal that grips to the surface on which it is painted. Hence, it can be used on concrete and it will grip to the liner, providing a completely waterproof seal.

The pond reconstructed, I turned to the problem of how to make a brick-built box into a filter system.

Although the box was gravity and pump fed, there was no evidence of any sort of media. As the pond had approximately two-thirds of its surface area covered with foliage, the water was clear. When using surface foilage, the suns UV rays are less likely to feed the algae bloom, as the leaves of various plants prevent the rays from getting to the water. Therefore, there was no need to use an Ultra Violet Clarifier (UVC). It just needed mechanical and biological filtration.

First the brick box had to be sealed. If the concrete rendering on the inside had previously been waterproofed, it wasn?t obvious. Once again, I used a plasticiser. The box then needed to be sectioned into three compartments. This was achieved by using plastic door runners, which were screwed into the sides and siliconed for a watertight seal, followed by Foamex, a plastic-coated, high-density foam, useful for its anti-rotting properties.

The first partition was cut so that the water would flow into that compartment and underneath the foam. The water would then rise into the next compartment, and then overflow into the third section, where a pump pushed the clean water back into the pond via a watercourse that was already in place.

Once the compartments had been created, it was a simple matter to add the filter media. For this job, I used Flocor as the initial medium. Flocor is a ribbed plastic, cut into lengths of about two inches, and is excellent for trapping the detritus of fish waste and rotting vegetation.

The second stage biological filtration was facilitated by a substance called Alfagrog. Alfagrog is actually a ceramic slag that has good porous qualities and is known to quickly build up a culture of the beneficial bacteria needed to break down waste products.

The final stage was the most protracted: to ensure that the pump within the filter system pushed out the clean water no faster or slower than the combined inflow of the gravity and pump feeds. And to ensure that the pump in the pond never clogged up (which would have made a mockery of any checks and balances I made) I used an Oase Aquamax, which although strangely shaped like an armadillo has the capacity to pass solids of 8-10 millimeters.

After what seemed like an eternity of fine-tuning the flow rates, everything was perfectly balanced. Three months later, the customer has not yet had to play with the pump flows and will not need to change the filter media for another three months.